This invention relates generally to oscillator circuits and more particularly to integrated oscillator circuits for use with an external capacitor.
Integrated oscillator or timer circuits that operate in conjunction with an external capacitor that is alternatively charged and discharged are well known in the art. One such circuit, for example, is the LM555 Timer circuit manufactured by National Semiconductor Corporation of Santa Clara, Calif., and described at pages 9-33 through 9-38 of the 1982 Linear Databook. The LM555 timer circuit provides both "one-shot" and free-running oscillator operational modes. The circuit used in the LM555 circuit uses a saturated bipolar transistor to discharge the external capacitor. The use of a saturated transistor is suitable for use in a stand-alone integrated circuit such as the LM555, but is unacceptable if the timer circuit is used as only one functional block out of many on the integrated circuit. The reason for this is that injected substrate current from the saturated transistor can de-bias and otherwise adversely affect the operation of the other functional blocks on the integrated circuit. In addition, the stored charge in the saturated device reduces the maximum operating frequency. An additional limitation of this device is that the timing ramp on the external capacitor is an exponential R-C charge/discharge waveform instead of a linear triangle waveform, unless an external current source is used.
Other circuits for achieving a linear charge/discharge waveform at the capacitor do so with a first constant charging current, and a second switched discharging current that is twice the magnitude of the charging current. These circuits are generally undesirable because of the associated high power consumption.
What is desired is an oscillator circuit having both one-shot and free-running operational modes, a linear charge/discharge capacitor waveform, and minimum power consumption requirements.